Studies during the second year of this grant have been concerned with a systematic evaluation of the effects of immunosuppressive agents on lymphocytes. Two major areas included: 1. the effects of immunosuppressants on subpopulations of lymphocytes and 2. the effect of cyclophosphamide on suppressor cells. In guinea pigs, T and B lymphocytes were measured in both peripheral blood and lymph node suspensions after immunosuppressive therapy. Following a single dose of cyclophosphamide (150 mgs/kg), both T and B cells were markedly reduced. The preferential toxicity of this drug for B cells appears to result from differences in the rates of recovery of the two cell populations; T cells were restored more rapidly. Chlorambucil caused less pronounced reductions. Neither 6 MP nor methotrexate, two phase specific drugs, caused depletion of either cell type. Hydrocortisone, in very large doses (400 mgs/kg), caused a modest but non-selective lympholysis. Additional studies with cyclophosphamide indicated that it profoundly depleted the lymphoid cells in the thymus; cortical recovery was appreciatively slower than peripheral B cells. Tests for activity of thymic cells inhibited responses to both PHA and pokeweed. Following cyclophosphamide, suppressor cell activity persisted. In a second study, the thymic suppressor cells in assays using lymph node cells stimulated either concanavalin A or specific antigen also proved resistent to cyclophosphamide. These systematic evaluations should permit development of immunosuppressive protocol which can more effectively inhibit undesired immune responses. BIBLIOGRAPHIC REFERENCES: Whiteside, T.L., Winkelstein, A., and Rabin, B.S. Immunologic characterization of chronic lymphocytic leukemia cells. Cancer, 1977, in press. Smith, W.I., Zidar, B.L., Winkelstein, A., Whiteside, T.L., Rabin, B.S., Shadduck, R.K., Zeigler, Z., Brietfield, B., Silverberg, J.H. and Rosenbach, L.M. Hairy cell leukemia: A case with B lymphocyte origin. Amer. J. Clin. Path., 1977, in press.